The invention is related generally to the field of electromagnetic induction resistivity well logging, and specifically to methods which require generation of a precisely-specified periodic signal downhole.
Background: Well Logging
Electromagnetic induction resistivity well logging instruments are well known in the art. Electromagnetic induction resistivity well logging instruments are used to determine the electrical conductivity of earth formations penetrated by a well bore. Measurements of the electrical conductivity are used for, among other things, inferring the fluid content of the earth formations. (Lower conductivity, or higher resistivity, is associated with hydrocarbon-bearing earth formations.)
The physical principles of electromagnetic induction resistivity well logging are described, for example, in H. G. Doll, Introduction to Induction Logging and Application to Logging of wells Drilled with Oil Based Mud, Journal of Petroleum Technology, vol. 1, p. 148, Society of Petroleum Engineers, Richardson Texas (1949). Many improvements and modifications to electromagnetic induction resistivity instruments have been devised since publication of the Doll reference, supra. Examples of such modifications and improvements can be found, for example, in U.S. Pat. Nos. 4,837,517, 5,157,605, 5,452,762, all of which are hereby incorporated by reference.
Background: Transverse Electromagnetic Induction Well Logging
In U.S. Pat. No. 5,781,436 issued to Forgang et al. a method for measuring the conductivity of earth formations penetrated by a well bore is described. The method of Forgang et al. involves selectively passing an alternating current through transmitter coils inserted into the wellbore. At least one of the transmitter coils has a magnetic moment direction different from the magnetic moment direction of the other ones of the transmitter coils. The alternating current includes a first and a second frequency where the amplitude of the alternating current at the first frequency has a predetermined relationship to the amplitude of the alternating current at the second frequency. Voltages induced in a receiver coil, having a magnetic moment along a direction substantially parallel to the magnetic moment direction of the transmitter coil through which the alternating current is passed, are selectively received. A difference in magnitudes between a component of the received voltage at the first frequency and a component of the received voltage at the second frequency is measured. The conductivity is calculated from the difference in the magnitudes at the induced frequencies as compared to the difference in magnitudes at the transmitted frequencies.
Background: Phase and Harmonic Distortions in Transmitted Signal
Because the difference in magnitudes at the received frequencies is compared to the difference in the amplitudes at the frequencies in the transmitted frequencies, the transmitted frequencies and their amplitudes must either be measured or carefully held to a known form.
However, precision electronics in a downhole sonde presents some unique challenges. Due to the net upward flow of heat in the Earth, the temperature can be very high in deep boreholes, and is commonly 175 degrees Celsius or higher. (Electronics are often enclosed in thermal insulation, so that the temperature of the electronics can be less than that in the borehole; but this enclosure merely slows the rate of change of temperature seen by the electronics.) Moreover, since the temperature varies with depth, it can change very significantly as a sonde is lowered during logging operations. Pressure also changes very significantly with depth.
This harsh environment presents problems for electronics. Some components, which would be stable at a known value while on the surface, are not stable inside the wellbore. Thus downhole generation of a precisely specified RF power output has proven difficult. The unpredictable change in component behavior can lead to phase and harmonic distortions in the transmitted signal. (A major cause of these distortions is non-linearity in the power amplifier.)
In some conventional induction logging techniques, the transmitted signal was simply sampled to precisely determine what transmitted signal had led to the received waveform. The properties of the rock formation were then sought to be derived from this information.
However, such methods are not satisfactory with the Forgang et al. method (U.S. Pat. No. 5,781,436). The non-linearity of the amplifier introduces frequency harmonics and beat frequencies into the signal, thereby corrupting it. Although the prior art methods have worked acceptably well for sinusoidal and even square wave signals passed through the transmitter, those methods have proven inadequate when dealing with the particular waveform desired to be generated by the logging instrument shown in the Forgang et al. patent. Therefore, providing precise control of the transmitted signal would be desirable.
Low Distortion Digitally Controlled Transmitter
The present application discloses a new method for generating precisely specified waveforms downhole. Thus instead of requiring refined filtering methods to recover the effects of propagation through the formation, the present invention provides a technique for precision transmittal downhole, whereby the transmitted waveforms can be optimized for easy analysis. This is moving in the opposite direction from the prior art, in which receiver optimization is emphasized more than transmission optimization.
The present application discloses a system and method to control a transmitter so that the transmitted signal is substantially identical to a specified, desired, periodic waveform such as would be required for well logging. In the context of well logging, the prior art uses a detection algorithm with a reference receiver, but the innovative teachings of the present application use a control algorithm to adjust the transmitted signal. The transmitted signal is sampled and compared to the desired signal. The template of the samples of the waveform to be transmitted is then modified so that amplitudes that are deficient are enhanced and those that are excessive are reduced. Because both sine and cosine components are adjusted, phase shifts in the output filter and power amplifier are corrected, as well as are gain errors. Harmonics introduced by the non-linearities in the transmitter are also corrected by the addition of harmonics of opposite polarities into the template. The adjustment of the template is performed for example using a novel combination of steepest descent adaptation of the sine and cosine amplitudes along with simultaneous determination of the parameters of the analog output filter and power amplifier.